The invention relates to a pressure screen for fibrous suspensions having a casing which accommodates a fixed cylindrical screen cage and a cylindrical rotor arranged for rotation therein, an inlet for the raw fibre stock in the region of one end of the rotor, an outlet for the separated specifically light waste matter of the suspension and a diluting water inlet in the region of a casing chamber bounding the other end of the rotor, and an outlet for the accepted stock communicating with the space external to the screen cage.
In the Wochenblatt fer Papierfabrikation, No. 15 (1973), page 567 there is disclosed such a pressure screen as a part of an apparatus for treating waste paper. This apparatus includes, underneath the pressure screen provided with a vertically arranged screen cage and a similarly arranged rotor, a so-called vortex cleaner, into which the specifically heavy particles of dirt are deposited and extracted through a so-called sediment trap. The raw fibre stock then flows upwardly at the centre of the vortex cleaner and from below into the downwardly open rotor designed as a rotating screen cage.
From a casing space above the rotor there is extracted the specifically light waste matter, which has been separated by the pressure screen, and diluting water is also introduced into this space of the casing.
This apparatus demands a comparatively high energy input because a considerable volume of the raw fibre stock must be accelerated and set in circulation. Accordingly the invention takes as its basic purpose the provision of a pressure screen, which, whilst demanding a low energy input, is particularly suitable for suspensions having a low content of the heavier waste matter but which is expecially efficient in removing the lighter dirt from the suspension. Starting from the basis of a pressure screen of the first specified type, this problem is solved by the invention in that the rotor is horizontally arranged and is closed at least at its inlet end and at the periphery, and that it has, at its outlet end, a device for promoting the circulation of the process material, and that the casing space bounding the outlet end of the rotor is provided at the top thereof with the outlet for the specifically lighter waste matter and at the bottom with an outlet for the specifically heavier waste matter. Thus, the apparatus according to the invention intentionally avoids using a vortex cleaner for separating the specifically heavier waste matter so that the volume of the suspension set into circulation is kept as low as possible, whereby driving energy is saved. At the same time however, by arranging the rotor horizontally, introducing the raw fibre stock in the direction of the rotor axis, as well as arranging the outlet for the specifically lighter waste material in the upper part, and arranging the outlet for the specifically heavier waste material in the lower part of the casing space bounding the rearward end of the rotor, there is ensured a reliable separation of the heavier particles from the specifically lighter waste material as well as the separation of the latter from the suspension, notwithstanding the fact that the apparatus according to the invention is of comparatively simple construction, consequently can be cheaply manufactured and is characterized by a compact design. By the introduction of diluting water and the high degree of turbulence, there is achieved in the casing space bounding the end of the rotor at the outlet side, an effective stripping of the lighter and heavier waste components from the acceptable fibre stock, which, in consequence of the introduced diluting water, is rinsed back into the annular space between the rotor and the screen cage and then leaves the pressure screen through the outlet for the accepted stock. It is worth mentioning that, as compared with the known apparatus first described, not only is the volume of suspension set into circulation considerably reduced by the avoidance of a vortex cleaner, but also as a consequence of using a closed rotor, it is only necessary that the circulating suspension shall fill the comparatively narrow annular space between the rotor and the screen cage in addition to the casing space bounding the end of the rotor at the outlet side.
In order that, in addition to the effect obtained by the centrally arranged inlet for the stock at the entrance of the casing, the conveyance of the so-called reject material into the casing space adjacent the outlet end of the rotor shall be further assisted, it is advisable to provide at the periphery of the rotor at least one clearing strip operating upon the screen cage, and which is arranged to be slightly inclined with respect to the rotor axis in such a manner that the moving rotor conveys the suspension in the direction of the outlet end of the rotor. However, the provision of such a clearing strip, or several of such clearing strips, improves not only the efficiency of the throughput, but also the screening effectiveness of the pressure screen.
Having regard to the flow conditions in the casing space adjacent the outlet end of the rotor, it is advisable that an outlet pipe for the specifically lighter waste material as well as an inlet pipe for the diluting water shall be joined to the casing in a direction which is substantially tangential and in correspondence with the direction of the circulation of the process material.